JPH0242805A - Amplifying circuit - Google Patents

Abstract

PURPOSE:To prevent an unnecessary signal output by providing a gain reducing circuit, which is operated when a gate circuit is turned off, in a circuit to amplify the output of the gate circuit, which is controlled by a gate signal, when the gate circuit is turned on. CONSTITUTION:A control signal to be read from a magnetic head 2 is amplified by a gate circuit operational amplifier 5. The output is supplied through a comparator 6 to a timing generating circuit 7 and a gate circuit 4 is turned on and off by the control signal from the circuit 7. The control signal from this circuit 7 is also supplied through an inverter 9 to a switch 8 of the gain reducing circuit. When the circuit 4 is turned off, the switch 8 is turned on and the gain reducing circuit goes to an operating condition. Thus, the level of a noise, etc., is lowered during an unnecessary period and malfunction is prevented from being generated.

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to an amplifier circuit, and more specifically, the present invention relates to an amplifier circuit that reads and reproduces control signals written on a control track of a magnetic tape or the like using a magnetic head. The present invention relates to an improvement of an amplifier circuit in a magnetic head readout circuit that can reduce noise components and accurately extract control signals, etc. in a magnetic head readout circuit.

[Prior Art] Conventionally, in video recording and playback devices (VTR), etc.,
While video signals are recorded on the video track, control signals are written on the control track.

The control signal is used to control the capstan servo system. For example, when playing a video signal, the mount roll head is positioned on the control track and read out, and the video head correctly scans over the recorded trunk. Used for control to reproduce good signals.

The magnetic head readout circuit that reproduces the control signal is
There are cases in which a window pulse is generated at a predetermined timing, and the readout waveform extracted accordingly is amplified and converted into a pulse by a waveform shaping circuit and then taken out. The amplifier circuit usually has a gate circuit inserted on the input side, and selectively amplifies the input signal (read waveform signal of the magnetic head) according to the timing of the gate signal.

[Problem to be solved] In this way, in an amplifier circuit that has a gate circuit on the input side, the input signal is cut during the period when the gate is closed (“OFF”). It remains in an amplified state regardless of gate operation. Therefore, even during the period when the gate is "OFF", noise corresponding to the gain appears on the output side of the amplifier circuit, including noise generated inside the amplifier circuit. In particular, in the amplifier circuit provided in the readout circuit of the magnetic head, the gain is set to a high value of, for example, several tens of dB to one hundred dB, so the next stage circuit is a waveform shaping circuit using a comparator or the like. In the case of a waveform pulsing circuit, the output noise of the amplifier circuit will be input to the comparator or level judgment circuit, etc., which may affect the judgment level and cause false detection or malfunction. Become.

Furthermore, in the magnetic head readout circuit described above, the gate is “
When the signal level is in the “OFF” state, a drooping phenomenon occurs in which the signal level drops to the negative side, and this
There is a risk that the signal will be amplified during the "FF" period and appear at the output 411, and will adversely affect the subsequent comparator or level determination circuit.

The present invention solves the problems of the prior art, and provides an amplifier circuit that does not generate unnecessary signals at the output of the amplifier circuit during the gate closed period (hereinafter referred to as gate "OFF" period). The purpose is to

[Means for Solving the Problems] The configuration of the amplifier circuit of the present invention to achieve such an object is to receive an input signal via a gate circuit, and to receive an input signal through a gate circuit, and to output an input signal during a period in which the gate circuit is open according to the gate signal. In an amplifier circuit that amplifies and outputs an obtained input signal, a gain reduction circuit is provided that reduces the gain of the amplifier circuit during a period when the gate circuit is closed in accordance with the gate signal.

[Function] In this way, by providing the gain reduction circuit and suppressing unnecessary output signals by reducing the gain of the amplifier circuit during the gate "OFF" period according to the gate signal, the gate is "OFF". "The noise level during the non-operating period in the "state" is lowered to prevent unnecessary signals from being input to the subsequent circuit. This prevents the subsequent circuit from malfunctioning and processes the input signal at an accurate level. This is to make it possible to do the following.

[Embodiment] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of a magnetic head readout circuit according to an embodiment to which an amplifier circuit of the present invention is applied.

In the figure, 1 is a magnetic tape set in a VTR, and 1a is a control track on which a control signal is written, for example, as a 30 Hz pulse signal. 2 is a control head (hereinafter referred to as magnetic head) that reads control signals from the control track 1a;
, and the 30Hz write pulse signal is applied to the magnetic tape 1.
The data is read out according to the scanning of the magnetic head 2 with respect to the data.

The control signal read by the magnetic head 2 is converted into an analog signal, with its direct current component cut off by a capacitor 3, and then input to an operational amplifier (OP) 5 via a gate circuit 4. This read signal is amplified by the operational amplifier 5 during the period when the gate circuit 4 is "ON", inputted to the comparator (COM) 6, waveform-shaped into a pulse signal, and outputted to the output terminal 10.

The control signal thus read and reproduced by the magnetic head 2 is output from the comparator 6, and is used as a signal to control the position where the video head scans the magnetic tape 1, etc., or is supplied to various other circuits. and used as a control signal.

This output pulse signal of the comparator 6 is further input to the timing generation circuit 7.

The timing generation circuit 7 receives the output pulse signal of the comparator 6, generates a window pulse for extracting the read signal of the magnetic head 2 at a predetermined timing according to the output pulse signal, and uses the window pulse as the next gate signal to generate the window pulse in the gate circuit 4. supply to.

Now, the gain of the operational amplifier 5 here is controlled according to the gate signal described above. Opamp 5 is
It is a non-inverting type amplification system, and in order to control its amplification factor, a switch circuit 8 is inserted in parallel with a feedback resistor Rf that determines the amplification factor.

Switching control of this switch circuit 8 is performed according to an output signal of an inverter 9 that receives a gate signal. Therefore, the operation of the switch circuit 8 is opposite to that of the gate circuit 4. In other words, during the gate open period with the gate signal (hereinafter referred to as the gate "ON" period), the "OF"
F” and turns “ON” during the gate “OFF” period when there is no gate signal.

In this way, when the switch circuit 8 is turned "ON" during a period other than the gate "ON" period, the input side and the output side of the operational amplifier 5 are short-circuited during this period, and the operational amplifier 5
The gain decreases to about 1 during this period. Therefore, the operational amplifier 5 operates as a buffer and its output voltage is fixed to the bias voltage at the input terminal. As a result, during the period when the gate circuit 4 is "OFF", a stable level signal with almost no noise appears at the output of the operational amplifier 5.

Note that the resistor R3 and the capacitor C are a bias circuit inserted in series between the reference level input side of the operational amplifier 5 and ground. Further, for the gate circuit 4° switch circuit 8, etc., an analog switch circuit is used, for example, and the operational amplifier 5. By using the inverter 9 and other components that can be integrated into an IC, the entire magnetic head reading circuit can be integrated into an IC.

As explained above, in this embodiment, an operational amplifier is used to connect the input side and the output side to reduce the gain to about 1. However, this invention does not necessarily apply when reducing the gain to a large extent like this. The present invention is not limited to this, and it is sufficient to reduce the gain to such an extent that even if noise is transmitted to the subsequent stage, it does not have an adverse effect.

Therefore, a resistor having a certain resistance value may be inserted in series with the switch circuit 8, and the connection form is not limited to that of the embodiment.

In addition, in the embodiment, control is performed to reduce the gain of the amplifier circuit using an inverter and a switch circuit.
Reducing the gain of the amplifier circuit is not limited to these methods.
For example, various gain reduction means can be used, such as lowering the power supply voltage of the operational amplifier, and the present invention is not limited to the circuit shown in the embodiment.

As can be understood from the explanation of "Effects of the Invention", the present invention includes a gain reduction circuit to reduce the gain of the amplifier circuit during the gate "OFF" period according to the gate signal. By suppressing unnecessary output signals by
The noise level during the non-operating period in the "FF" state is reduced to prevent unnecessary signals from being input to the subsequent circuit, making it difficult for the subsequent circuit to malfunction and processing input signals at accurate levels. can do.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a magnetic head readout circuit according to an embodiment to which an amplifier circuit of the present invention is applied. DESCRIPTION OF SYMBOLS 1... Magnetic f-pu, la... Control truck, 2... Magnetic head, 4... Gate circuit, 5...
- Operational amplifier, 6... Comparator, 7... Timing generation circuit, 8... Switch circuit, 9... Inverter.

Claims (1)

[Claims] (1) In an amplifier circuit that receives an input signal via a gate circuit, amplifies and outputs the input signal obtained during the period when the gate circuit is open according to the gate signal, and outputs the amplified signal during the period when the gate circuit is closed. An amplifier circuit comprising: a gain reduction circuit that reduces the gain of the amplifier circuit according to the gate signal.